This project proposes to study the morphogenetic expression of determination in chick embryos. The patterned emergence of parts of the embryo is well exemplified by the complex morphogenesis of the mesoblast into metameric somites. The very early events that elicit and pattern the somites will be studied by light and electron microscropy, microsurgical intervention, analysis of quail-chick chimeras, and biochemical means. Specifically, this study includes 1) evaluation of the morphological relationship between somitomeric pattern in the head process and subsequent branchiomeric development and cranial neural crest migration, 2) morphological examination of the transformation of primitive streak mesenchyme into axially patterned chordamesoderm, 3) evaluation of the role of extracellular matrix in the establishment and maintenance of metameric pattern, 4) elucidation of the morphological role of the primary organizer region in eliciting axial pattern from the primitive streak, and 5) development of a quail cell surface marker for scanning electron microscopic identification of cells in experiments utilizing quail-chich chimeras. It is hoped that a critical evaluation of these developmentally crucial events may suggest mechanisms by which embryonic tissues interact to become different from one another and provide information to our understanding of normal and abnormal development of carniofacial structures in particular.